Why is regression testing important in software development, especially after code changes, bug fixes, or new feature additions?

Introduction / Context:
Regression testing is a core practice in software quality assurance. Whenever code changes are made, whether to fix defects, add features, or refactor existing modules, there is a risk that previously working functionality may break. Regression testing addresses this risk by re running selected test cases or suites to confirm that older behaviour remains correct. Understanding why regression testing is important helps teams appreciate its role in continuous integration, agile development, and stable releases.

Given Data / Assumptions:
- The question asks about the importance of regression testing, not about specific tools.
- We assume a project where code changes happen regularly.
- Existing functionality is already covered by test cases that previously passed.
- There is a risk that new changes may introduce side effects or integration issues.

Concept / Approach:
Regression testing is based on the idea that software systems are interconnected. A small change in one module can affect other modules in unexpected ways. After changes, teams re execute a set of test cases that cover critical flows and previously fixed defects. If these tests still pass, there is increased confidence that the system has not regressed. The correct answer must state that regression testing checks that new changes have not broken existing functionality that used to work correctly.

Step-by-Step Solution:
Step 1: Recall that regression testing is executed after changes such as bug fixes, enhancements, or refactoring.Step 2: Recognise that its purpose is not to verify only the new feature, but to ensure that unaffected areas still behave correctly.Step 3: Understand that regression suites often include high priority, high risk, and frequently used scenarios.Step 4: Review the options and choose the one that explicitly states that regression testing verifies that existing functionality has not been broken by recent changes.Step 5: Select option A because it captures the essence of regression testing in clear and accurate terms.

Verification / Alternative check:
Imagine an e commerce site where developers add a new discount feature on the checkout page. After implementing the change, they run unit tests and confirm that the new discount logic works for the specific scenarios coded. QA then runs regression tests, including add to cart, payment processing, order history viewing, and invoice generation, all of which worked correctly before the change. If these tests pass, the team gains confidence that the discount feature did not break existing flows. If some tests fail, regression testing has successfully revealed unintended side effects that must be fixed before release.

Why Other Options Are Wrong:
Option B claims that regression testing replaces unit tests, which is incorrect. Unit tests are written and run by developers at a finer granularity, while regression tests focus on integrated behaviour. Option C focuses on hardware temperature and network bandwidth, which are operational metrics, not the main goal of regression testing. Option D discusses documentation formatting, which is unrelated to verifying the stability of software behaviour after code changes.

Common Pitfalls:
Common pitfalls include running regression tests too infrequently, such as only before major releases, which can allow regressions to accumulate. Another mistake is allowing the regression suite to grow without review, leading to slow or redundant tests that are rarely executed. To avoid these issues, teams should regularly review and prioritise regression tests, focusing on high value scenarios. Automated regression suites integrated into continuous integration pipelines provide fast feedback, helping teams detect regressions soon after they are introduced.

Final Answer:
Correct answer: Because it verifies that recent changes have not unintentionally broken existing functionality that previously worked correctly